Chitosan is a highly versatile biopolymer commonly derived from crustacean (i.e., crabs, shrimps, etc.) shell chitin, but is also found in the cell walls of certain fungi. It has found numerous applications in various industries due to its unique molecular characteristics as well as its ability to form fibers, films, hydrogels, and coatings, all with antimicrobial properties. Additionally, as a polymeric material of biological origin, chitosan is biodegradable, biocompatible, and has low toxicity. The molecules below show the respective structures for chitosan, chitin, and cellulose.

Paper is a fibrous, hydrophilic substrate; it can absorb liquid penetrants such as water, grease, and oil. Furthermore, uncoated paper allows for the easy passage of moisture. The main reason for this easy passage of moisture is the spaces between the interwoven fibers of paper that consist of countless air voids. It is these air voids and micropores within the cell walls of fibers that determine the porosity of the paper, which in turn is influenced by fiber refining.
Barrier coatings are applied to the surface of paper to decrease the porosity of the paper by filling the air voids between the fibers, coating the micropores in the fiber walls, and changing the surface chemistry of the fibers to make them resistant to fluid wetting and liquid absorption. Barrier coatings that provide rigidity and water resistance in corrugated board and paperboard products are widely used in food packaging and HVAC (Heating, Ventilation, and Air Conditioning) cooling pads for agricultural markets.
For agricultural applications, one major market is the use of barrier-coated board as part of the evaporative cooling system in barns. For these applications, water resistance and rigidity is obtained through the application of extruded polyethylene, wax, phenolic resins, or acrylic resins. The type of chemistry used depends on the degree of water resistance needed and the specific regulatory requirements that must be met. Furthermore, for HVAC, including agricultural applications, a certain degree of water absorption/wicking is required in order to produce an evaporative cooling effect on the surface of the cooling pads. This market currently consumes 4.8 M tons/yr of barrier-coated board. Phenolic resins provide stiffness to the wet paper substrate and represent 13-15% of barrier coating applications, which is estimated to be 0.6-0.7 million tons annually. However, current phenolic resin-based coatings fail quickly and their use precludes recycle of the paperboard. Therefore, there is an opportunity for natural biopolymers, with none of the VOC (volatile organic compound) and recyclability concerns that phenolic polymers have, to displace these materials along with petroleum-based barrier coating chemistries as an environmentally friendly alternative while at the same time providing enhanced structural integrity.
Although the availability of chitin/chitosan in the biosphere is estimated at 10 trillion tons, mostly from crustacean shells, only a minute fraction—2000 tons annually—is currently utilized, mostly for high purity applications in food, pharmaceutical, and biomedical products mainly due to high costs of up to $1000/kg due to processing and recovery difficulties. These current methods to generate chitin/chitosan generate significant quantities of wastes, and suffer from raw material supply interruptions, and product quality and yield inconsistencies.
The potential benefits of using chitosan-based coatings may be significant based on the renewability of this biopolymer in addition to the relative amounts of the material required for a desired application. If the demand for chitosan becomes necessary, soybean oil fatty acids and other lower cost bio-based chemistries such as lignin sulfonates could be used to supplement and produce more chitosan. Using these types of naturally occurring sources to produce chitosan could potentially further reduce costs of using chitosan by enabling more application at a competitive price.
There accordingly remains a need to find both a consistent and efficient bio-based, environmentally friendly alternative to phenolic resins for use as coatings and/or additives to paper. There also remains a need for an antimicrobial coating and/or additive to be used in paper-based evaporative cooling media while offering beneficial functional properties such as wicking ability, moisture resistance, abrasion resistance, and durability/mechanical stability in saturated conditions. Further, any bio-based materials used in these applications should be able to be reprocessed to separate and recover the coating materials and paper to recycle the resulting products for other applications.